The day of reckoning for US politics approaches as Americans prepare to go to the polls.

On Tuesday, US voters will decide between President Donald Trump's take-no-prisoners rhetoric and the Democratic Party's super-charged campaign to end the Republicans' monopoly in Washington and state legislatures across the nation.

There are indications that a so-called "blue wave" may help Democrats seize control of at least one chamber of US congress.

However, two years after an election that proved pollsters and political commentators wrong, nothing is certain on the eve of the first nationwide elections of the Trump presidency.

All 435 seats in the US house of representatives are up for re-election. Another 35 US senate seats are in play, as well as almost 40 gubernatorial votes and the balance of power in virtually every state chamber.

While he is not on the ballot, Mr Trump has acknowledged that the 2018 mid-term elections represent a referendum on his presidency.

Should Democrats win control of the house of representatives, as strategists in both parties suggest is likely, they could derail Mr Trump's legislative agenda for the next two years.

US President Donald Trump is bolstering the border. Photo: Getty Images

Perhaps more importantly, they would also win subpoena power to investigate the president's many personal and professional mis-steps.

Tuesday's elections will also test the strength of a Trump-era political realignment defined by evolving divisions among voters by race, gender and especially education.

U.S. President Donald Trump rallies with supporters at Middle Georgia Regional Airport in Macon, Georgia, U.S. November 4, 2018. REUTERS/Jonathan Ernst

Mr Trump's Republican coalition is increasingly becoming older, whiter, more male and less likely to have a college degree. Democrats are relying more upon women, people of colour, young people and college graduates.

The political realignment, if there is one, could re-shape US politics for a generation.

Just five years ago, the Republican National Committee (RNC) reported that the party's very survival depends upon attracting more minorities and women. Those voters have increasingly fled Mr Trump's Republican Party, turned off by his chaotic leadership style and xenophobic rhetoric. Blue-collar men, however, have embraced the unconventional president.

A nationwide poll released on Sunday by NBC News and The Wall Street Journal details the depth of the demographic shifts.

Mr Trump has made migration a central part of his US mid-term election campaign (AP)

Democrats led with likely African-American voters (84% to 8%), Latinos (57% to 29%), voters between the ages of 18-34 (57% to 34%), women (55% to 37%) and independents (35% to 23%).

On the other side, Republicans led with voters between the ages of 50 and 64 (52% to 43%), men (50% to 43%) and whites (50% to 44%). And among white men without college degrees, Republicans led 65% to 30%.

Democrats hope to elect a record number of women to congress. They are also poised to make history with a number of LGBT candidates and Muslims up and down the ballot.

Former president Barack Obama seized on the differences between the parties in a late scramble to motivate voters across America.

"One election won't eliminate racism, sexism or homophobia," Mr Obama said during an appearance in Florida. "It's not going to happen in one election. But it'll be a start."

Mr Trump has delivered a very different closing argument, railing against Latin American immigrants seeking asylum at the US border.

With a walking caravan of immigrants weeks away from reaching the border, Mr Trump dispatched more than 5,000 troops to the region. The president also said soldiers would use lethal force against migrants who throw rocks, before later reversing himself.

The charged atmosphere is expected to drive record turnout in some places, but on the eve of the election, it is far from certain which side will show up in the greatest numbers.

While Mr Trump is prepared to claim victory if his party retains senate control, at least one prominent ally fears that losing even one chamber of congress could be disastrous.

"If they take back the house, he essentially will become a lame-duck president, and he won't win re-election," said Amy Kremer, a Tea Party activist who leads the group, Women For Trump.

"They'll do anything and everything they can to impeach him," she said.

Powerful Democratic forces are already pushing for Mr Trump's impeachment, even if Democratic leaders are not yet ready to go that far.